[PATCH] sectiongroup support

Shankar Easwaran shankare at codeaurora.org
Sat Mar 15 22:02:37 PDT 2014

Hi Nick,

This is the overall design that I am working on to support 
reading/resolving section groups in the YAML reader.

a) Read all atoms
b) any atoms with group-parent references are moved to 
AtomList<lld::DefinedAtom> _groupChildAtoms;
c) There would also be a map _groupChildMap that has a key from 
groupChild to groupParent, this is to support (d)
d) If there is a reference to a groupChild from another child(whose 
parent is different from the previous group child), the groupChild 
reference is changed to refer to a undefined atom for the groupChild
e) if there is a reference to a groupchild that doesnot have a 
group-parent reference, and if the reference is not to a atom that is 
undefined, create an undefinedatom and point the group child.

*We could also do something where all groupchild references refer to 
undefined atoms.*

*There seems to be a caveat even after everything looks sane.

a) There is a funny case of what do we do when the group child is a 
local symbol ? So if there is a reference to a group child whose scope 
is local, we cannot create a undefined reference to the group child right ?
*b) The problem is we call the RoundTripYAML reader/writer and the 
RoundTripNative reader/writer, which happens to be called after the 
resolver is called, which would create undefined references to atoms!.**
*I am concerned on just the above cases ?

Do we want to have a different mode in the YAML reader when called from 
RoundTripYAMLPass not to create undefined atoms ?

Am I missing something ?


Shankar Easwaran

On 3/14/2014 8:29 PM, Nick Kledzik wrote:
> On Mar 14, 2014, at 4:37 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:
>> On 3/14/2014 5:52 PM, Nick Kledzik wrote:
>>> On Mar 13, 2014, at 8:24 PM, Shankar Kalpathi Easwaran <shankarke at gmail.com> wrote:
>>>>   I dont think the multithreaded model of processing all the object files first is going to work with the ELF linker, as resolution happens in the way input files are processed from command line.
>>>>   This is what I see with the GNU linker :-
>>>>   a) The groups are being resolved as and when input files are processed by the resolver. We cannot delay processing the group members after the resolver has processed all files.
>>>>   b) Consider the below example :-
>>> ...
>>>>   With the command line ld foo.o bar.o libarchive.a, when symbols from bar.o are processed, **global-f3 becomes a undefined symbol** and pulls the global-f3 from libarchive.a
>>>>   How do we handle all these cases with lld ?
>>>> http://llvm-reviews.chandlerc.com/D2961
>>> Given that the ELF linker needs those semantics, it seems that the Resolver and symbol table building must be done serially, in command line order.  The object files could still be parsed in parallel, but the results would need to be buffered and added to the symbol table in command line order.  Given that, here are two possible algorithms:
>>> 1) Groups are like static libraries
>>> a) group child atoms are not returned in the defined() atoms collection, but group parents are.
>>> b) files are added to the symbol table in command line order
>>> c) when a group parent atom is added to the symbol table, its name is checked to see if that group name has been seen yet by the symbol table.  If not, the group name is added and all its children are also added.  If the group name has already been seen, then the group’s child remain unseen by the symbol table.
>>> Problem: uses of any atom in a group from outside the group would require the ELF reader to synthesized UndefinedAtom and have that be referenced. In a way this is good because it enforces that groups can be removed.
>> Thanks for the great suggestion. We could follow the option (1) above, as the group childs are not seen by the rest of the linker until the group is added.
>> Are you saying that the group childs are internal to the lld::File, and can be only referenced from the group-child references in the parent atom (contentType:kindGroupComdat) ?
> Without adding some other mechanism for getting a list of atoms, yes, that is how to do it.
>> To support groups in the core/yaml reader, the user has to be explicit in creating undefined/defined atoms for the group child members ?
> I supposed you could have the yaml reader detect and automatically create the UndefinedAtoms.  I’m not sure which is easier.
> Also, this is going to require making lots of error laden test cases in ELF assembly and see how the existing ELF linkers handle them.  For instance if group g1 contains a function that calls foo, and that is the only call to foo in the object file, when that file is loaded, if g1 is not used (because another group named g1 already exists), will the undef for foo get added to the global symbol table?  If not, then somehow undefined symbols also need to be associated with groups.
> -Nick

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